1. Field of the Invention
This invention relates generally to power supplies and more particularly concerns DC-to-DC power converters.
2. Background Art
A common form of power converter is the DC-to-DC converter, which converts an input DC voltage to an output DC voltage having a desired value. Since the principal form of line power is AC, some type of AC-to-DC power supply is usually used to produce the requisite DC input voltage for the DC-to-DC converter. Where several different DC output voltages are required, several DC-to-DC converters, operating to produce the different output voltages, are connected in common to the same input DC voltage.
There are a number of different topologies for DC-to-DC converters. In many cases, such a converter includes a semiconductor switch which is turned on and off to couple energy from the DC input to an inductor in the converter. This energy is transferred from the inductor to the DC output either during the on time or the off time of the switch, depending upon the converter topology. Common DC-to-DC converter topologies include the buck (or forward) converter, the buck-boost (or flyback) converter, and the boost converter topologies.
As will be noted below with regard to an exemplary embodiment, the invention will find advantageous use in a buck, or forward, converter, but may also be used advantageously in other converter topologies.
In a conventional buck converter, a semiconductor switch is connected between the DC supply input and an inductor, which is in turn connected to the output. The junction between the switch and the inductor is coupled to circuit common, or ground, by a diode (termed a "flywheel" diode), which is normally reverse biased and non-conductive when the semiconductor switch is closed. Ordinarily a capacitor is connected between the output and circuit common. A typical inductor in this form of converter has an inductance on the order of 100 microhenries, and a typical capacitor has a capacitance in the order of 500 microfarads.
During normal operation of the conventional buck converter, the switch is closed, impressing the input voltage, less the output voltage, across the inductor. This causes the current in the inductor to increase, charging the output capacitor while also delivering current to any load connected at the output.
When the switch is turned off, the voltage at the connection between the switch and the inductor falls until the diode becomes forward biased. Current then flows through the diode and the inductor with decreasing amplitude until the switch is again closed and the cycle repeated.
In such prior art buck converters, it is advantageous to operate the converter at as high a frequency as possible, in order to reduce the size of the reactive components in the circuit. Typical prior art buck converters might operate at frequencies up to about twenty kilohertz. There have been upper limits to the operating frequency of prior art buck converters due to switching losses in the semiconductor switches in the converters.
Switching losses occur when the series semiconductor switch in a buck converter is turned on and off because of the finite time required for the current to start and stop flowing in the device. As the switch is turned on, current flowing through the device causes the voltage at the junction between the device and the inductor to rise to the level of the input voltage, producing dissipation equal to the instantaneous product of the current through the device and the voltage across the device. Similarly, as the series switch is turned off, the simultaneous presence of a large voltage across the switch and a large current through the switch produces dissipation. These switching losses in the semiconductor switch increase with increasing frequency of operation since the number of switching excursions per unit time increases with frequency.
In the past, power FET's have been used as series switches in buck converters in order to improve efficiencies. The use of such a power FET is advantageously because it eliminates minority carrier storage time and permits faster switching. The FET drive circuitry is also more efficient than that for a bipolar transistor.
A similar advantage in elimination of minority carriers is obtained if the diode in the converter is replaced with an FET. The user of an FET in place of the flywheel diode in prior buck converters has, however, called for critical timing of the FET turn-on and turn-off to avoid overlapping conduction of the series switch FET and the flywheel FET and to avoid "dead time" when neither device is conducting. Overlapping conduction of the FET series switch and the flywheel FET greatly increases dissipation in the circuit. Dead time causes parasitic diodes in the FET's to turn on, which in turn produces additional dissipation due to the presence of stored charge in one FET when the other FET is turned on.
Not only does switching loss become more of a problem as operating frequency is increased, but the critical timing requirements for a two FET system also become more difficult to meet in order to avoid overlapping conduction or dead time as the time between switching events becomes shorter. Switcing loss and loss due to timing errors are both directly proportional to frequency, as stated earlier, while the difficulty of maintaining tight tolerances on critical timing parameters to minimize timing increases as the switching period becomes shorter.